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United States Patent |
5,578,815
|
Nakase
,   et al.
|
November 26, 1996
|
Bias circuit for maintaining a constant potential difference between
respective terminals of more than one avalanche photodiode
Abstract
A bias circuit for applying a bias voltage to an avalanche photodiode APD2
for detecting light comprises a first diode APD1, a power supply V.sub.H
connected to the first diode APD1, for applying a voltage to make the
diode in breakdown between an anode and a cathode of the first diode APD1,
and a constant voltage circuit V2 connected to the avalanche photodiode
APD2 for detecting light, for applying a voltage difference of a breakdown
voltage generated between the anode and the cathode of the first diode
APD1 minus a constant voltage to the avalanche photodiode. The constant
voltage is substantially independent from current flowing in the avalanche
photodiode APD2 for detecting light to the avalanche photodiode.
Inventors:
|
Nakase; Shigeki (Hamamatsu, JP);
Nakamura; Shigeyuki (Hamamatsu, JP);
Ohta; Tsuyoshi (Hamamatsu, JP)
|
Assignee:
|
Hamamatsu Photonics K.K. (Hamamatsu, JP)
|
Appl. No.:
|
272071 |
Filed:
|
July 8, 1994 |
Foreign Application Priority Data
Current U.S. Class: |
250/214R; 250/214C; 327/514; 327/538 |
Intern'l Class: |
G01J 001/42 |
Field of Search: |
250/214 C,238,214 R
327/513,514,538
|
References Cited
U.S. Patent Documents
4181863 | Jan., 1980 | Parker | 327/513.
|
4236069 | Nov., 1980 | Laughlin | 250/214.
|
4292514 | Sep., 1981 | Ohtomo | 250/214.
|
4599527 | Jul., 1986 | Beaudet et al. | 327/513.
|
4945227 | Jul., 1990 | Jones et al. | 250/214.
|
5194727 | Mar., 1993 | Johnson et al. | 250/214.
|
Foreign Patent Documents |
60-111540 | Jun., 1985 | JP.
| |
60-180347 | Sep., 1985 | JP.
| |
244218 | Feb., 1990 | JP.
| |
Primary Examiner: Westin; Edward P.
Assistant Examiner: Lee; John R.
Attorney, Agent or Firm: Cushman Darby & Cushman, L.L.P.
Claims
What is claimed is:
1. A photodetecting circuit comprising:
(a) a first avalanche photodiode for detecting light, said first avalanche
photodiode having a first breakdown voltage and a first cathode;
(b) a second avalanche photodiode having a second breakdown voltage and a
second cathode, said second breakdown voltage being within 100.+-.20% of
said first breakdown voltage; and
(c) a constant voltage circuit connecting said first cathode and said
second cathode, wherein a potential of said second cathode is higher than
a potential of said first cathode, and wherein a difference in potential
between said first cathode and said second cathode is maintained constant
by said constant voltage circuit.
2. A photodetecting circuit according to claim 1, wherein said second
avalanche photodiode has an anode, said photodetecting circuit further
comprising a power supply connected to said second avalanche photodiode
for applying a voltage between said anode and said second cathode to make
said second avalanche photodiode breakdown.
3. A photodetecting circuit according to claim 1, wherein said constant
voltage circuit comprises a Zener diode having an anode and a cathode,
said cathode of said Zener diode being connected to said second cathode,
and said anode of said Zener diode being connected to said first cathode
of the first avalanche photodiode.
4. A photodetecting circuit according to claim 1, further comprising:
a transistor having an emitter, a base and a collector, said emitter being
connected to ground, said collector being connected to an anode of said
first avalanche photodiode; and
an operational amplifier having two input terminals and an output terminal,
said output terminal being connected to said base of said transistor, one
of said input terminals being connected to said output terminal via a
condenser and to ground via a variable resistor, and the other of said
input terminals being connected to said collector via a first resistor and
to ground via a second resistor.
5. A bias circuit for applying a bias voltage to a first avalanche
photodiode having a first cathode, said first avalanche photodiode
detecting light and having a first breakdown voltage, said bias circuit
comprising:
(a) a second avalanche photodiode having a second cathode, said second
avalanche photodiode having a second breakdown voltage that is within
100.+-.20% of said first breakdown voltage; and
(b) a constant voltage circuit connecting said first and second cathodes,
wherein a potential of said second cathode is higher than a potential of
said first cathode, and wherein a difference in potential between said
first cathode and said second cathode is maintained constant by said
constant voltage circuit.
6. A bias circuit according to claim 5, wherein said second avalanche
photodiode has an anode, said bias circuit further comprising a power
supply connected to said second avalanche photodiode for applying a
voltage between said anode and said second cathode to make said second
avalanche photodiode breakdown.
7. A bias circuit according to claim 5, wherein said constant voltage
circuit comprises a Zener diode having an anode and a cathode, said
cathode of said Zener diode being connected to said second cathode, said
anode of said Zener diode being connected to said first cathode of the
first avalanche photodiode.
8. A bias circuit according to claim 5, further comprising:
a transistor having an emitter, a base and a collector, said emitter being
connected to ground, said collector being connected to an anode of said
first avalanche photodiode; and
an operational amplifier having two input terminals and an output terminal,
said output terminal being connected to said base of said transistor, and
one of said input terminals being connected to said output terminal via a
condenser and to ground via a variable resistor, and the other of said
input terminals being connected to said collector via a first resistor and
to ground via a second resistor.
9. A bias circuit for applying a bias voltage to a plurality of avalanche
photodiodes for detecting light, said avalanche photodiodes each having a
breakdown voltage corresponding thereto, comprising:
(a) a first avalanche photodiode having an anode and a cathode, a breakdown
voltage of said first avalanche photodiode being within 100.+-.20% of the
respective breakdown voltages of said plurality of avalanche photodiodes;
and
(b) a plurality of constant voltage circuits, each connecting a cathode of
one of said plurality of avalanche photodiodes to said cathode of said
first avalanche photodiode, a potential at said cathode of said first
avalanche photodiode being higher than a potential at any of said cathodes
of said plurality of avalanche photodiodes, potential differences between
said cathode of said first avalanche photodiode and said cathodes of each
of said plurality of avalanche photodiodes each being maintained constant
by said plurality of constant voltage circuits, respectively.
10. A bias circuit according to claim 9, wherein said first avalanche
photodiode has an anode, said bias circuit further comprising a power
supply connected to said first avalanche photodiode for applying a voltage
between said anode and said cathode of said first avalanche photodiode to
make said first avalanche photodiode breakdown.
11. A bias circuit for applying a bias voltage to a first avalanche
photodiode for detecting light, said first avalanche photodiode having a
first anode and a first breakdown voltage, said bias circuit comprising:
(a) a second avalanche photodiode having a second anode, said second
avalanche photodiode having a second breakdown voltage within 100.+-.20%
of said first breakdown voltage; and
(b) a constant voltage circuit connecting said first and said second
anodes, wherein a potential at said second anode of said second avalanche
photodiode is lower than a potential at said first anode of said first
avalanche photodiode, a potential difference between said first anode and
said second anode being maintained constant by said constant voltage
circuit.
12. A bias circuit according to claim 11, wherein said second avalanche
photodiode has a cathode, said bias circuit further comprising a power
supply connected to said second avalanche photodiode for applying a
voltage between said second anode and said cathode of said second
avalanche photodiode to make said second avalanche photodiode breakdown.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a bias circuit for driving an avalanche
photodiode with high multiplication factor.
2. Related Background Art
An avalanche photodiode (APD) is a semiconductor photodetector which has
high photodetection sensitivity and high speed of response utilizing the
avalanche multiplication. The APD is used to perform the photodetection
with high sensitivity. However, each APD has an operating characteristic
which varies according to temperature during operation. As a temperature
compensating circuit for the APD, circuits disclosed in "Japanese Patent
Laid-open No. Shou 60-111540 (111540/1985)", "Japanese Patent Laid-open
No. Shou 60-180347 (180347/1985)", and "Japanese Patent Laid-open No. Hei
2-44218 (44218/1990)" have been known.
SUMMARY OF THE INVENTION
The inventors of the present application found the fact that the difference
between the voltage at which the APD showed a constant multiplication
factor and the breakdown voltage was substantially constant. The present
invention was developed based on this discovery. In the case of using a
circuit of the present invention, the photodetection can be performed with
higher stability to temperature as compared with a conventional circuit
which is disclosed in "Japanese Patent Laid-open No. Hei 2-44218
(44218/1990)" (see FIG. 5-FIG. 8).
The present invention relates to a bias circuit for applying a bias voltage
to an avalanche photodiode for detecting light. This bias circuit
comprises a first diode, a power supply connected to the first diode, for
applying a voltage between an anode and a cathode of the first diode to
make the first diode in breakdown, and a constant voltage circuit
connected to the avalanche photodiode for detecting light, for applying a
voltage difference of a breakdown voltage generated between the anode and
the cathode of the first diode minus a constant voltage to the avalanche
photodiode. The constant voltage is substantially independent from current
flowing in the avalanche photodiode for detecting light.
In a view of temperature compensation (compensation for the temperature
dependence of the APD gain versus voltage relationship), the first diode
is preferably an avalanche photodiode, and the first diode preferably has
the similar structure as the avalanche photodiode for detecting light. The
similar structure means that the breakdown voltage of one avalanche
photodiode is within a range of 100.+-.20% of the breakdown voltage of the
other avalanche photodiode. This constant voltage circuit can be achieved
using, e.g., a Zener diode. A cathode of the Zener diode is connected to a
cathode of the first diode, and an anode of the Zener diode is connected
to the cathode of the avalanche photodiode for detecting light.
The Zener diode operates in the breakdown region by applying a reverse bias
voltage. The voltage generated at both ends of the ideal Zener diode does
not depend on current flowing in the avalanche photodiode for detecting
light. In other words, the constant voltage circuit generates a voltage
substantially independent from current flowing in the avalanche photodiode
for detecting light. In a case that the current flowing in the avalanche
photodiode for detecting light varies .+-.50% and the voltage generated by
the constant voltage circuit varies in a range of .+-.20%, the constant
voltage circuit generates a voltage "substantially" independent from the
current flowing in the avalanche photodiode for detecting light.
Further, a bias circuit of the present invention comprises a first diode, a
power supply for applying a reverse voltage to make the diode in breakdown
between an anode and a cathode of the first diode, and a constant voltage
circuit connected between an anode of the avalanche photodiode for
detecting light and ground, for generating a constant voltage
substantially independent from current flowing in the avalanche photodiode
for detecting light.
In a view of temperature compensation, the first diode is preferably an
avalanche photodiode and has the similar structure as the avalanche
photodiode for detecting light.
The constant voltage circuit may comprises a Zener diode, and a cathode of
the Zener diode may be connected to the cathode of the first diode, and an
anode of the Zener diode may be connected to the cathode of the avalanche
photodiode for detecting light.
Further, the constant voltage circuit comprises an operational amplifier
the output of which is connected to an anode of the avalanche photodiode
for detecting light, a first resistor connected between a non-inverting
input of the operational amplifier and the output of the operational
amplifier, a second resistor connected between a non-inverting input of
the operational amplifier and ground, a condenser connected between the
inverting input of the operational amplifier and the output of the
operational amplifier, and a third resistor connected between the
inverting input of the operational amplifier and ground.
The constant voltage circuit further comprises a transistor connected
between the output of the operational amplifier and the anode of the
photodiode for detecting light, and a base of the transistor is connected
to the output of the operational amplifier, an emitter to ground, and a
collector to the anode of the photodiode for detecting light. The constant
voltage circuit may further comprise a variable transistor connected
between the third resistor and ground. One end of the variable resistor is
kept at a predetermined potential.
The present invention also relates to a photodetection circuit for
outputting a signal corresponding to incident light. A photodetection
circuit comprises a first diode, a power supply connected to the first
diode, for applying a reverse voltage between an anode and a cathode of
the first diode to make the diode in breakdown, a plurality of avalanche
photodiodes for detecting light connected to a cathode of the first diode,
and a constant voltage circuit for generating a constant voltage
substantially independent from current flowing in the avalanche photodiode
for detecting light, connected between the cathode of the first diode and
a cathode of the avalanche photodiode for detecting light, or between an
anode of the avalanche photodiode for detecting light and ground.
In a view of temperature compensation, the first diode is preferably an
avalanche photodiode and has the similar structure as the avalanche
photodiode for detecting light. The constant voltage circuit may comprise
a Zener diode the cathode of which is connected to the first diode and the
anode of which is connected to the cathode of the avalanche photodiode for
detecting light.
The present invention will become more fully understood from the detailed
description given hereinbelow and the accompanying drawings which are
given by way of illustration only, and thus are not to be considered as
limiting the present invention.
Further scope of applicability of the present invention will become
apparent from the detailed description given hereinafter. However, it
should be understood that the detailed description and specific examples,
while indicating preferred embodiments of the invention, are given by way
of illustration only, since various changes and modifications within the
spirit and scope of the invention will become apparent to those skilled in
the art from this detailed description.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a circuit diagram of basic structure of the present invention.
FIG. 2 is a graph showing measurement results of a breakdown voltage
Vb.sub.1 of APD1, a breakdown voltage Vb.sub.2 of APD2, and a temperature
coefficient related to a multiplication factor M of APD2 or others.
FIG. 3 is a circuit diagram showing one embodiment of a bias circuit using
a Zener diode ZD.
FIG. 4 is a circuit diagram showing one embodiment of a bias circuit in
which a voltage difference between a breakdown voltage and a bias voltage
can be adjusted.
FIG. 5 is a graph showing temperature dependence of a multiplication factor
M of a bias circuit of the present invention (solid line) and a
conventional bias circuit (dotted line) (the multiplication factor is 20
at room temperature).
FIG. 6 is a graph showing temperature dependence of a multiplication factor
M of a bias circuit of the present invention (solid line) and a
conventional bias circuit (dotted line) (the multiplication factor is 50
at room temperature).
FIG. 7 is a graph showing temperature dependence of a multiplication factor
M of a bias circuit of the present invention (solid line) and a
conventional bias circuit (dotted line) (the multiplication factor is 100
at room temperature).
FIG. 8 is a graph showing temperature dependence of a multiplication factor
M of a bias circuit of the present invention (solid line) and a
conventional bias circuit (dotted line) (the multiplication factor is 200
at room temperature).
FIG. 9 is a circuit diagram showing one example of bias circuit structure
in which a plurality of APDs operate at the same multiplication factor
with high stability.
FIG. 10 is a circuit diagram showing one example of bias circuit structure
in which a plurality of APDs operate at the same multiplication factor
with high stability.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The embodiments of the present invention will be explained with reference
to the drawings. The inventors of the present application have developed
the photodetection circuits for detecting optical signals which are stable
against the change of temperature using a first APD for sensing
temperature and a second APD for detecting an optical signal the
characteristics of which are substantially the same as that of the first
APD. When two avalanche photodiodes which have the similar structure are
made of the same material, their characteristics are theoretically matched
but practically not. Note that the similar structure means that the
breakdown voltage of one avalanche photodiode is within 100.+-.20% of the
breakdown voltage of the other avalanche photodiode.
The inventors of the present application experimented many times and found
that when the voltage difference (Vi.sub.2 =Vb.sub.1 -V2) of the breakdown
voltage (Vb.sub.1) of the first APD minus the substantially constant
voltage (V2) was applied to the second APD circuit, the temperature
characteristic of the multiplication factor (M) of the second APD was
drastically improved. In other words, in the circuit according to the
present invention, the first APD and the second APD satisfy a relation of
Vb.sub.1 -Vi.sub.2 =constant value (V2). The second APD circuit comprises
the second APD. Note that in a case of the magnification factors of the
first APD and the second APD exceeding 50, the temperature characteristic
of the magnification factor of the second APD is drastically improved.
A constant voltage circuit for generating a potential difference which is
substantially independent from the current flowing into the second APD is
connected between the second APD circuit and the first APD to subtract the
substantially constant voltage (V2) from the breakdown voltage (Vb.sub.1)
of the first APD, and then the voltage (Vi.sub.2) is applied to the second
APD circuit. Further, in the bias circuit according to the present
invention, the voltage by which the first diode APD1 is in breakdown may
be applied to the first diode APD1, and the cathode of the first diode
APD1 and the cathode of the second diode APD2 may be connected, and the
constant voltage circuit may be connected between the anode of the second
diode APD2 and ground. A constant voltage source using a Zener diode or an
operational amplifier is one example of such a constant voltage circuit.
It is well-known that "constant voltage circuit" generates a voltage which
is completely not independent from a circuit connected thereto. In a case
that the quantity of currents flowing into an avalanche photodiode APD2
for detecting light and the voltage generated by the constant voltage
circuit varies within .+-.20%, the constant voltage circuit V2 generates a
voltage which does "substantially" not depend on current flowing into the
avalanche photodiode APD2 for detecting light.
A bias circuit for an avalanche photodiode according to the present
invention was developed based on the above findings.
FIG. 1 shows a circuit diagram of a bias circuit according to one
embodiment of the present invention. The bias circuit uses two APDs the
characteristics of which are similar. The first APD1 is used for sensing
temperature, not for causing light to be incident. The second APD2 is used
for detecting an optical signal. One feature of the bias circuit is that
the voltage difference VB=Vi.sub.2 =Vb.sub.1 -V2 of the voltage Vb.sub.1
(breakdown voltage Vb.sub.1) which is defined by the potential of the
cathode of the first diode APD1 minus the constant voltage (V2) which is
independently controllable against the current flowing in the second APD2
is applied to the cathode of the second APD2 (input of the second APD
circuit).
The anode of the first APD1 is grounded. The cathode of the first APD1 is
connected to a node A of FIG. 1. The anode of a power supply V.sub.H is
connected to the node A through a constant current source I.sub.S1. The
cathode of the power supply V.sub.H is grounded. The current I.sub.s flows
into the node A. The constant voltage circuit V2 is connected between the
node A and a node B. The constant voltage circuit V2 can decrease the
potential at the node B V2 (volts) lower than the potential at the node A.
In other words, the potential difference between the node A and the node B
is substantially constant (V2) not depending on the current flowing in the
second APD2. The potential difference between the node A and the node B
can be adjusted by the constant voltage circuit V2 if necessary.
A resistor R1 for dividing current is connected between the node B and
ground. The cathode of the second APD2 is connected to the node B. The
anode of the second APD2 is connected to the node C. A load resistor
R.sub.L of the second APD2 is connected between the node C and ground. A
condenser C is connected between the node C and the output OUT. The second
diode APD2, the load resistor R1, and the condenser C constitute the
second APD circuit. The cathode of the second APD2 is an input of the
second APD circuit.
In the following explanation, it is defined that Vm.sub.1, Vm.sub.2,
Vi.sub.2, Vb.sub.1, and Vb.sub.2 denote a bias voltage of the first APD1,
a bias voltage of the second APD2, an input voltage of the second APD
circuit, a breakdown voltage of the first APD1, and a breakdown voltage of
the second APD2, respectively.
The operation of the circuit shown in FIG. 1 will be explained. The
constant current Is is applied from the power supply V.sub.H to the first
diode APD1. The voltage (V.sub.H volts) enough to make the first diode
APD1 in breakdown is applied between the anode and cathode of the first
diode APD1. Accordingly, the current Is is applied to the cathode of the
first diode APD1, so that the first diode APD1 is in breakdown. The
breakdown voltage (Vb.sub.1) generated at both ends of the first APD1
(between the anode and cathode) is defined by a potential difference
between the potential Vb.sub.1 at the node A and the ground potential
(0V).
Since the constant voltage circuit V2 is connected between the node A and
the node B, the potential VB (Vi.sub.2) at the node B is decreased voltage
V2 lower than the potential Vb.sub.1. Consequently, the potential
VB=Vb.sub.1 -V2 is applied to the cathode of the second diode APD2. That
is, the voltage VB=Vi.sub.2 =Vb.sub.1 -V2 is applied to the second APD
circuit.
Assuming the voltage at the load resistor R.sub.L is V.sub.L, the voltage
Vm.sub.2 =Vi.sub.2 -V.sub.L =Vb.sub.1 -(V2+V.sub.L) is applied between the
anode and cathode of the second diode APD2. Accordingly, the voltage
difference Vm.sub.2 of the breakdown voltage Vb.sub.1 of the first diode
APD1 minus the constant voltage V2 which does not depend on the current
flowing in the second diode APD2 is applied to the second APD circuit.
The first diode APD1 and the second diode APD2 are contained in the same
package. In other words, the first diode APD1 and the second diode APD2
are placed under the same circumstances, so that the diode APD1 and the
diode APD2 have the same temperature.
The bias voltage Vm.sub.2 is a high voltage so that the multiplication
factor M of the second diode APD2 is large enough to be a multiplication
factor M (50 or above). The multiplication factor M of the second diode
APD2 is large enough, so that the photodetection can be performed with
high sensitivity using this circuit.
As the breakdown voltage Vb.sub.1 of the first diode APD1 varies, the bias
voltage Vm.sub.2 =Vb.sub.1 -(V2+V.sub.L) applied to the second diode APD2
varies in accordance with the change of the voltage Vb.sub.1. In other
words, the bias voltage Vi.sub.2 applied to the second APD circuit varies
the same amount of change of the breakdown voltage Vb.sub.1 of the first
diode APD1. Consequently, the temperature dependence of the multiplication
factor M of the second diode APD2 for detecting an optical signal is
suppressed, and the temperature dependence of the output of the second APD
circuit is suppressed. The photodetection which is stable against the
change of temperature can be performed with use of the circuit shown in
FIG. 1.
This is based on the following reasons. First, the characteristics of the
avalanche photodiodes which would be used as the first diode APD1 or the
second diode APD2 were evaluated. FIG. 2 is a graph showing bias voltage
dependence of a temperature coefficient (V/.degree. C.) of each avalanche
photodiode, and breakdown voltage Vb.sub.1 dependence of a temperature
coefficient (V/.degree. C.) of the first diode APD1 and breakdown voltage
Vb.sub.2 dependence of a temperature coefficient (V/.degree. C.) of the
second diode APD2 in the circuit shown in FIG. 1.
The temperature of each APD varied from -15.degree. C. to +55.degree. C. at
a step of 10.degree. C. (total of 7 points). The relation between the
temperature coefficient (V/.degree. C.) and the bias voltage (V) required
for obtaining the desired multiplication factor M (M=10, 20, 50, 100) of
the APD was examined at every temperature.
An APD which had the breakdown voltage Vb.sub.1 of 215V at room temperature
among APDs (type S2383) manufactured by Hamamatsu photonics k.k. was used
as the first diode APD1. An APD which had the breakdown voltage Vb.sub.2
of 220V at room temperature among APDs (type S2383) manufactured by
Hamamatsu photonics k.k. was used as the second diode APD2. The measuring
wavelength .lambda. of light was 800 nm, and the measuring power of light
was 1 nW.
The horizontal axis denotes a bias voltage (V) and the vertical axis
denotes a temperature coefficient (V/.degree. C. It is understood from
FIG. 2 that the breakdown voltage Vb.sub.1 of the first APD1 (shown as
black squares in FIG. 2), the breakdown voltage Vb.sub.2 of the second
APD2 (shown as black triangles in FIG. 2), and the temperature coefficient
of the multiplication M of the second APD2 (shown as white squares, white
triangles, white circles, and asterisks in FIG. 2) varied as the bias
voltages (Vm.sub.1, vm.sub.2) applied to the first APD1 and the second
APD2 varied.
The evaluation of the APD characteristics shown in the graph of FIG. 2 is
done by the inventors of present application for the first time.
It is considered from the graph of FIG. 2 that there is some relation
between the temperature coefficient and the bias voltage. In the
conventional bias circuit techniques for the avalanche photodiode, it was
considered that "a ratio of the breakdown voltage and the bias voltage is
constant". In other words, the temperature coefficient also varies in a
proportion of the ratio of the breakdown voltage and the bias voltage.
Supposing this consideration is true in a high multiplication factor (M=50
or above) region, a line connecting the plotted symbols should be
approximated by a straight line A passing through the origin.
However, it is apparent from FIG. 2 that a line connecting the plotted
symbols cannot be approximated by a ling passing through the origin if the
breakdown voltage is divided by the resistor R.sub.1 and the ratio of the
breakdown voltage V and the bias voltage is constant.
In particular, in this multiplication factor region (M=50 or above), since
the change of the multiplication factor M is large as compared with the
change of the bias voltage, an error of the multiplication factor M
becomes large and the stability of the sensitivity against temperature
becomes worse.
On the other hand, in the bias circuit of the present invention, the first
APD1 the characteristics of which is similar as that of the second APD2 is
in breakdown, and the bias voltage of the breakdown voltage of the first
APD1 minus the constant voltage is applied to the APD2, so that the
stabilization of the multiplication factor M can be achieved by simple
circuit.
The multiplication factor M varies according to temperature, and as
apparent from the graph of FIG. 2, in the case of a large multiplication
factor M (M=50 or above), lines connecting plotted symbols for each
multiplication factor show the same tendency, and these lines coincide
when shifted in a horizontal direction.
Consequently, the bias circuit, which compensates the change of the
characteristics of the multiplication factor caused by the change of
circuit temperature by making the voltage difference between the breakdown
voltage of the first diode APD1 and the bias voltage applied to the second
diode APD2 to be constant, can suppress the temperature dependence much
lower as compared with the circuit in which the ratio of the breakdown
voltage and the bias voltage is constant.
In FIG. 3, the constant voltage circuit V.sub.2 shown in FIG. 1 which gives
the constant voltage difference between the breakdown voltage Vb.sub.1 of
the first APD1 and the bias voltage Vm.sub.2 of the second APD2 is
achieved with a Zener diode. The constant current source I.sub.S comprises
a high voltage source (not shown) and a resistor (not shown) connected
between the high voltage source and the first APD1. The constant current
source I.sub.S is connected between the cathode of the first diode APD1
and ground. The anode of the first diode is grounded. The cathode of the
Zener diode ZD is connected to a node A to which the constant current
source I.sub.S and the cathode of the first diode APD1 are connected. The
anode of the Zener diode ZD is connected to a node B. The resistor
R.sub.21 is connected between the node B and ground.
In this circuit, the first diode APD1 and the second diode APD2 are also
under the same thermal condition, and the first diode APD1 is used as a
temperature sensor, and the first diode APD1 is kept in a breakdown
condition.
The bias voltage of the constant Zener voltage V.sub.Z minus the breakdown
voltage Vb.sub.1 of the first diode APD1 is applied to the APD2 to operate
the second diode APD2 with the high multiplication factor M (note that
R.sub.21 is a resistor for dividing current). When the temperature varies,
as the breakdown voltage of the first diode APD1 varies, the voltage
applied to the second diode APD2 varies. The temperature coefficient of
the bias voltage of the second diode APD2 having the constant
multiplication factor is substantially the same as the temperature
coefficient of the breakdown voltage of the first diode APD1. The
multiplication factor of APD2 is high and kept constant.
FIG. 4 is a circuit diagram showing a circuit which is able to adjust the
voltage difference between the breakdown voltage and the bias voltage. A
cathode of a power supply V.sub.H is grounded. An anode of the power
supply V.sub.H is connected to a node A. A resistor R31 is connected
between the node A and a node B. A cathode of a first diode APD1 is
connected to the node B. The anode of the first diode APD1 is grounded. A
corrector of a transistor Tr31 is connected to the node A. A base of the
transistor Tr31 is connected to the node B.
An emitter of the transistor Tr31 is connected to a cathode of a second
diode APD2. An anode of the second diode APD2 is connected to the node C.
A constant voltage circuit 120 is connected to the node C. A resistor 32
is connected between the node C and the node D. A resistor R33 is
connected between the node D and ground. A corrector of a transistor Tr32
is connected to the node C. A base of the transistor Tr32 is connected to
a node E. A non-inverting input of an operational amplifier Q31 is
connected to the node D. A condenser C13 is connected between an inverting
input of the operational amplifier Q31 and the node E.
An output of the operational amplifier Q31 is connected to the node E. The
inverting input of the operational amplifier Q31 is connected to a node F.
A resistor 34 is connected between the node F and a volume VR31 which is a
variable resistor. One end of the variable resistor VR31 is connected to a
reference voltage source 122 and the other end is grounded. A condenser C1
is connected between the node C and the output OUT.
In the same way as the circuit shown in FIG. 1, when the voltage is applied
to the first diode APD1 by the power supply V.sub.H, the first diode APD1
operates in the breakdown region. The voltage of the cathode of the first
diode APD1 is buffered and applied to the cathode of the second diode
APD2. The constant voltage circuit 120 is connected to the anode of the
second diode APD2. Consequently, the voltage difference between the
breakdown voltage of the first diode APD1 and the output voltage of the
constant voltage circuit 120 is applied to the second diode APD2 as a bias
voltage.
The constant voltage circuit 120 is a circuit in which the reference
voltage from the reference voltage source 122 is divided by the volume
VR31 and this divided voltage is applied to the anode of APD2 from an
amplifier which comprises the operational amplifier Q31 and the transistor
Tr32. The output voltage of the circuit 120 can vary by the volume VR31,
and the magnification factor M of the second diode APD2 is adjusted and
set by the volume VR31. In FIG. 4, the leakage current of the second diode
APD2 flows into the emitter and collector of the transistor TR32. In the
case of very small leakage current, the stable operation cannot be
achieved. In such a case, a resistor for dividing current is connected in
parallel to the second diode APD2.
The temperature dependence of the bias circuit shown in FIG. 4 was
evaluated. FIGS. 5-8 are graphs showing the temperature dependence of the
multiplication factor M of the second diode APD2 shown in FIG. 4. In FIGS.
5-8, the solid lines show the multiplication factor M of the APD2 for
detecting light in the case of using the bias circuit of the present
invention of FIG. 4, and the dotted lines show the multiplication factor M
of the APD2 for photodetection in the case of using the conventional bias
circuit disclosed in "Japanese Patent Laid-open No. Hei 2-44218
(44218/1990)".
The characteristics of the first diode APD1 and the second diode APD2 are
similar as the characteristics of the APD shown in FIG. 2, These
evaluations were conducted under the condition that the wavelength
.lambda. of light for measurement was 800 nm and that the power of light P
was constant, and that the temperature was in a range of -20.degree. C. to
+60.degree. C.
FIG. 5 is a graph showing experimental results which were conducted by
adjusting the bias voltage of the APD2 for detecting a signal and setting
the multiplication factor M of the second diode APD2 for detecting a
signal to 20 at 25.degree. C.
FIG. 6 is a graph showing experimental results which were conducted by
adjusting the bias voltage of the APD2 for detecting a signal and setting
the multiplication factor M of the second diode APD2 for detecting a
signal to 50 at 25.degree. C.
FIG. 7 is a graph showing experimental results which were conducted by
adjusting the bias voltage of the APD2 for detecting a signal and setting
the multiplication factor M of the second diode APD2 for detecting a
signal to 100 at 25.degree. C.
FIG. 8 is a graph showing experimental results which were conducted by
adjusting the bias voltage of the APD2 for detecting a signal and setting
the multiplication factor M of the second diode APD2 for detecting a
signal to 200 at 25.degree. C.
As apparent from these results, the bias circuit of the present invention
can suppress the changes of the multiplication factor M of the second
diode APD2 to very low and improve its temperature characteristic. In
other words, the bias circuit, which performs the temperature compensation
of the multiplication factor by fixing the voltage difference between the
breakdown voltage of the first diode APD1 and the bias voltage of the
second diode APD2 to be constant, is superior to the bias circuit, which
performs the temperature compensation by fixing the ratio of the breakdown
voltage of the first diode APD1 and the bias voltage of the second diode
APD2, in the temperature compensation of the multiplication factor.
FIG. 9 shows a bias circuit in which a plurality of APDs operate with high
stability and the same multiplication factor. A cathode of a first diode
(APD for temperature compensation) APD1 is connected to an anode of a
power supply V.sub.H. A resistor R31 is connected between the cathode of
the first diode APD1 and the anode of the power supply V.sub.H. An anode
of the first diode APD1 is grounded. A cathode of the first diode APD1 is
connected to anodes of a plurality of equivalent power supplies V2.sub.1,
V2.sub.2, V2.sub.3 . . . through a buffer amplifier 140. Cathodes of a
plurality of second diodes (APDs for detecting light) APD2.sub.1,
APD2.sub.2, APD2.sub.3, and APD2.sub.4 are connected to cathodes of the
power supplies V2.sub.1, V2.sub.2, V2.sub.3 . . . , respectively. An input
of a circuit (transimpedance amplifier) 130.sub.1, 130.sub.2 and 130.sub.3
for converting current to voltage is connected to each anode of the second
diode. Optical signals detected by the second diodes APD2 are outputted
from outputs OUT1, 2, 3 . . . of the circuits 130.sub.1, 130.sub.2,
130.sub.3 . . . , respectively.
In this circuit, the diode APD1 is made to operate in breakdown region by
the power supply V.sub.H and the resistor R31, and its cathode voltage is
amplified by the buffer amplifier 140 the gain of which is 1 and applied
to the APD2.sub.1, APD2.sub.2, APD2.sub.3 . . . .
The voltage applied to each APD2.sub.1, APD2.sub.2, APD2.sub.2, and
APD2.sub.4 is adjusted individually by the equivalent constant voltage
sources V2.sub.1, V2.sub.2, V2.sub.3 . . . (in the same way as FIG. 3,
constituted by a high voltage source, and a resistor) because the bias
voltage of each APD for a constant multiplication factor is different from
each other. The anodes of the APD2.sub.1, APD2.sub.2 and APD2.sub.3 are
connected to the inverting inputs of the operational amplifiers in the
circuits 130.sub.1, 130.sub.2, 130.sub.3 . . . , respectively. The output
current of each APD appears at the output of the circuit as the voltage
expressed by the product of the output current of the APD and the resistor
R.sub.1, R.sub.2, R.sub.3 . . . . As described above, in this circuit, the
change of the multiplication factor caused by the change of temperature is
also suppressed, and the sensitivity is adjusted only by setting the
multiplication factor with V2.sub.1, V2.sub.2, V2.sub.3 . . . .
FIG. 10 shows a bias circuit which can adjust the bias voltage to be
applied to a plurality of the second diodes APD2.sub.1, APD2.sub.2,
APD2.sub.3 . . . in the same way as the one shown in FIG. 4. Amplifiers
132.sub.1, 132.sub.2, 132.sub.3 . . . are connected to these second diodes
APD2.sub.1, APD2.sub.2, and APD2.sub.3 . . . , respectively. In this bias
circuit, the APD1 is made to operate in breakdown by the power supply
V.sub.H and the resistor R31, and the cathode voltage is directly applied
to the cathodes of the APD2.sub.1, APD2.sub.2, and APD2.sub.3.
On the other hand, the anodes of the second diodes APD2.sub.1, APD2.sub.2,
APD2.sub.3 . . . are connected to the inverting inputs of the operational
amplifiers 132.sub.1, 132.sub.2, 132.sub.3 . . . , respectively. The
potential of the non-inverting inputs of the operational amplifiers
132.sub.1, 132.sub.2, 132.sub.3 . . . can be adjusted by the variable
resistors V.sub.1, V.sub.2, V.sub.3 . . . . The potential of the inverting
input and the non-inverting input of each operational amplifier 132.sub.1,
132.sub.2, 132.sub.3. . . are operated to be qual, so that the voltage
difference between the breakdown voltage of the first diode APD1 and the
voltage set by each variable resistor (volume) V.sub.1, V.sub.2, V.sub.3 .
. . is applied to the second diode APD2 as a bias voltage.
Since the cathodes of the plurality of the second diodes APD2.sub.1,
APD2.sub.2, APD2.sub.3 . . . and the cathode of the first diode APD1 are
connected to the same node, this bias circuit can easily be formed on the
same silicon substrate. Further, the voltage applied to each second diode
APD2.sub.1, APD2.sub.2, APD2.sub.3 . . . is needed to be adjusted
individually since the bias voltage for each second diode APD2.sub.1,
APD2.sub.2, APD2.sub.3 . . . to generate the constant multiplication
factor is different.
The temperature coefficient of each second diode APD2.sub.1, APD2.sub.2,
APD2.sub.3 . . . is substantially constant, so that the bias voltage
Vm.sub.2 can be set the constant voltage lower than the breakdown voltage
of the first diode APD1 only by adjusting the variable resistors VR1 and
VR2 connected to the non-inverting input of each operational amplifier
132.sub.1 and 132.sub.2. Consequently, the stability of the bias circuit
is drastically improved and the plurality of APDs are easily operated.
Thus, the bias circuit of the present invention can operate with high
stability by setting only the multiplication factor, and the adjustment of
every temperature coefficient is not required. Further, in the case of the
bias circuit operating at the constant voltage difference between the bias
voltage and the breakdown voltage, the stability of the bias circuit is
superior in a high multiplication factor (>100) region, and the bias
circuit can easily be used in the multiplication factor of 300-500.
Furthermore, in a multi-configuration, a process of adjusting a product
can drastically be reduced and the change of the multiplication factor of
each pixel is suppressed, and the APD can easily be utilized in a very
feeble light region.
As described above, according to the present invention, the difference
between the bias voltage and the breakdown voltage is kept at constant.
Consequently, in the case that the difference between the voltage at which
the avalanche photodiode shows a high multiplication factor and the
breakdown voltage is constant, the bias circuit can operate at high
multiplication factor although the temperature varies. Therefore,
photodetection can be performed by simple circuit, high sensitivity and
high stability against the change of temperature, using avalanche
photodiodes.
From the invention thus described, it will be obvious that the invention
may be varied in many ways. Such variations are not to be regarded as a
departure from the spirit and scope of the invention, and all such
modifications as would be obvious to one skilled in the art are intended
to be included within the scope of the following claims.
The basic Japanese Application No. 170289 filed on Jul. 9, 1993 is hereby
incorporated by reference.
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